1. Field of the Invention
The present invention relates to an optical pickup apparatus, condensing optical system, and optical element which condense a light beam onto the information recording surface of an optical information recording medium.
2. Description of the Prior Art
Recently, as short-wavelength red lasers have been put into practice, DVDs (Digital Versatile Disks) have been commercially available, which are high-density optical information recording media (also called optical disks) almost equal in size to CDs (Compact Disks) and having large capacities.
In addition to the above CDs and DVDs, optical disks with various specifications which differ in light source wavelength and protective substrate thickness have been commercialized, including a CD-R and RW (rewritable compact disks), VD (Video Disk), MD (Mini Disk), MO (Magneto Optical disk), and the like.
As the wavelengths of semiconductor lasers become shorter, research and development are under way to develop high-density optical disks including, for example, a high-density optical disk (Blue-ray Disc) with a protective substrate thickness of about 0.1 mm which uses a blue-violet semiconductor laser light source having a wavelength of about 400 nm and an objective lens whose image-side numerical aperture (NA) is increased to as large as about 0.85, and a high-density optical disk (HD) with a protective substrate thickness of about 0.6 mm which uses an objective lens having a numerical aperture (NA) of about 0.65. Hereinafter, each of these high-density optical disks is referred to as HD.
There has been proposed an optical pickup apparatus which converges light beams of different wavelengths to the information recording surfaces of a plurality of disks by using one objective lens, i.e., has so-called compatibility.
In general, in an optical pickup apparatus having compatibility, since the respective types of optical information recording media (e.g., a DVD and CD) vary in protective substrate thickness and the wavelength of a light beam to be used, it is difficult to satisfy the sine condition for correcting off-axis coma while correcting spherical aberration and wave aberration for both a DVD and a CD.
In addition, it is difficult to perform “temperature characteristic correction” of correcting a change in the wave aberration of an optical element with a temperature change for the respective types of optical information recording media. Since the wave aberration worsens as the focus position of a condensed light spot deviates due to a change in on-axis chromatic aberration or spherical aberration (chromatic aberration) occurring in an optical element when the wavelength of a light beam changes, it is difficult to perform chromatic aberration correction of correcting the chromatic aberration of an optical element with respect to a change in the wavelength of a light beam for the respective types of information recording media.
Conventionally, an optical pickup apparatus having compatibility has been known, which has a ring-like or grating-like diffraction structure formed on the surface of an objective lens or the surface of an optical element placed separately from the objective lens (see, for example, patent reference 1: Japanese Unexamined Patent Publication No. 2001-147367 and patent reference 2: Japanese Unexamined Patent Publication No. 2002-277732).
The apparatus disclosed in patent reference 1 is designed to perform the above types of correction and the like by using diffracted light beams of the same order obtained by applying the diffraction effects of the diffraction structures provided on the two surfaces of the objective lens to two types of light beams of different wavelengths.
Patent reference 2 discloses an optical pickup apparatus including an objective lens on which a diffraction structure (zone plate) is comprised of stepped portions.
In this apparatus, a diffraction effect is applied to a light beam of a wavelength of 650 nm, of two types of wavelengths, i.e., 650 nm and 780 nm, of light beams collimated by a collimator lens, by using the convex surface shape of an objective lens and the aspherical shapes of the two surfaces of the objective lens, thereby converging the light beam to the recording surface of a DVD. A diffraction effect is applied to a light beam of a wavelength of 780 nm by using the zone plate to converge the light beam to the recording surface of a CD-R in an aberration corrected state. Note that the zone plate has wavelength selectivity and is designed to apply no diffraction effect to a light beam of a wavelength of 650 nm.
In the case of patent reference 1, however, since the same diffraction structure is used for the respective light beams, it is difficult to perform the above independent types of correction for the respective light beams.
In the case of patent reference 2, since the zone plate is configured to apply a diffraction effect to only one light beam while applying no diffraction effect to the other light beam, an aberration correction function obtained by using diffracted light can be provided for only one light beam, and the above respective types of correction and the like using diffracted light cannot be performed for the other light beam. Patent reference 2 also discloses a technique of providing zone plates on the two surfaces, i.e., the incident and exit surfaces, of an objective lens. In this case as well, however, the zone plates on the two surfaces are configured to apply diffraction effects to only one light beam. Therefore, this technique cannot solve the problem of insufficient correction for the other light beam.